The present invention is directed to rotor track and balance methods, in general, and more particularly, to a method for determining a minimal set of adjustments for the blades of a rotor supported by a supporting structure for reducing rotation-induced vibration in the supporting structure at least one harmonic rotation order or for reducing track deviation while maintaining structure vibration over at least one harmonic order within an acceptable limit.
In the past, rotor track and balance methods were limited to minimizing rotation-induced vibration in a supporting structure, like a helicopter fuselage, for example, at only the fundamental harmonic order of the blade rotation which was considered the most problematic to the pilot and passengers. If the optimization process relied solely on vibration effects at the fundamental frequency, it could lead to a non-unique blade adjustment solution because blade track deviation was not taken into consideration. Therefore, to avoid ambiguities in the adjustment process, track measurements were considered a necessary addition to vibration processing to order to have an acceptable rotor track and balance procedure.
The concept of the U.S. Pat. No. 4,937,758 (the xe2x80x9c""758 Patentxe2x80x9d), entitled xe2x80x9cMethod and Apparatus For Reducing Vibration Over The Full Operating Range Of A Rotor And A Host Devicexe2x80x9d and issued on Jun. 26, 1990 proved that rotor track and balance could be performed uniquely without the need of blade track measurements, i.e. with vibration measurement data alone. The ""758 Patent teaches minimizing rotation-induced vibration in the helicopter fuselage not only at the problematic fundamental rotation order, but at all relevant harmonic orders in order to uniquely specify the best combination of rotor blade adjustments of all of the available types, i.e. tabs. pitch rods and weights. While this approach lead to a very smooth operating aircraft with minimal track deviation, it spread the number of blade adjustments needed over all of the available types and the rotating blades. Practically, the approach resulted in a high maintenance solution especially with aircraft having six and seven blades. It was recognized that in many cases most of the benefit could be realized with only a few adjustments, the remainder of recommended adjustments affording only a marginal improvement.
Accordingly, a rotor track and balance method which leads to an improved maintenance solution from a practical standpoint while keeping vibration and track deviation at acceptable limits is desired. One such solution should keep the number of blade adjustments recommended to a minimum just so long as the resulting vibration and track conditions while not optimum are reduced to desired levels. Such a method(s) is(are) taught in the following specification.
In accordance with one aspect of the present invention, a method of determining a minimal set of rotor blade adjustments for a rotor comprising the steps of: providing measurements of vibration at predetermined locations on a supporting structure of the rotor; resolving the vibration measurements into a corresponding set of Fourier coefficients of vibration for at least one of the locations, each Fourier coefficient of vibration of the set corresponding to a harmonic order of rotor rotation; establishing a vibration model of said supporting structure comprising Fourier coefficients of unit vibration influence for the at least one location, the Fourier coefficients of unit vibration influence corresponding to harmonic orders of rotor rotation; establishing a set of Fourier coefficients of adjustment for at least one blade adjustment type, the set of Fourier coefficients of adjustment corresponding to a set of blade adjustments associated with each blade adjustment type, tile Fourier coefficients of adjustment corresponding to harmonic orders of rotor rotation; selecting a Fourier coefficient of adjustment from each blade adjustment type set; establishing a relationship by which vibration of said supporting structure may be calculated at harmonic orders of rotor rotation from the Fourier coefficients of adjustment, the Fourier coefficients of unit vibration influence and the Fourier coefficients of vibration measurements; determining a value of each selected Fourier coefficient of adjustment that provides a desired virtual reduction of vibration of the supporting structure at the harmonic order of the selected Fourier coefficient of adjustment using the vibration relationship; determining a group of minimal sets of virtual blade adjustments for each type based on the determined value of the corresponding selected Fourier coefficient of adjustment; and determining a minimal set of virtual blade adjustments from the group of each type based on its affect on the vibration relationship at at least one harmonic order of rotor rotation other than the harmonic order of the selected Fourier coefficient of adjustment.
In accordance with another aspect of the present invention, a method of determining a minimal set of rotor blade adjustments for reducing track deviation of the rotating blades while maintaining vibration of a supporting structure of the rotor over at least one harmonic order of rotor rotation within an acceptable limit, said method comprising the steps of: providing measurements of vibration at predetermined locations on the supporting structure; resolving the vibration measurements into a corresponding set of Fourier coefficients of vibration for at least one of the locations, each Fourier coefficient of vibration of the set corresponding to a harmonic order of rotor rotation; providing measurements of track of the rotating blades; resolving the track measurements into corresponding Fourier coefficients of track that correspond to harmonic orders of rotor rotation; establishing a vibration model of the supporting structure comprising Fourier coefficients of unit vibration influence for the at least one location, the Fourier coefficients of unit vibration influence corresponding to harmonic orders of rotor rotation; establishing a track model for the rotating blades comprising Fourier coefficients of unit track influence, the Fourier coefficients of unit track influence corresponding to harmonic orders of rotor rotation; establishing a set of Fourier coefficients of adjustment for at least one blade adjustment type, the set of Fourier coefficients of adjustment corresponding to a set of blade adjustments associated with each blade adjustment type, the Fourier coefficients of adjustment corresponding to harmonic orders of rotor rotation; selecting a Fourier coefficient of adjustment from each blade adjustment type set; establishing a relationship by which vibration of the supporting structure may be calculated at harmonic orders of rotor rotation from the Fourier coefficients of adjustment, the Fourier coefficients of unit vibration influence and the Fourier coefficients of vibration measurements; establishing a relationship by which track of the rotating, blades may be calculated at harmonic orders of rotor rotation from the Fourier coefficients of adjustment, the Fourier coefficients of unit track influence and the Fourier coefficients of track measurements; determining a value of each selected Fourier coefficient of adjustment that provides a desired virtual reduction of track deviation of the rotating blades at the harmonic order of the selected Fourier coefficient of adjustment using the track relationship; determining a group of minimal sets of virtual blade adjustments for each type based on the determined value of the corresponding selected Fourier coefficient of adjustment; and determining a minimal set of virtual blade adjustments from the group of each type based on its affect on the vibration relationship at at least one harmonic order of rotor rotation other than the harmonic order of the selected Fourier coefficient of adjustment.